O evento cruzado seminal que iniciou a Marvel em seu caminho para os mega-crossovers foi: As Guerras Secretas publicado em 1984.
Embora os Vingadores fossem conhecidos por criar mega-histórias com uma coleção diversificada de heróis, foram as Guerras Secretas e as Guerras Secretas II que provaram a viabilidade de transições tanto de maneira histórica quanto financeira.
A Wikipédia resume bem:
The series was conceived by Marvel Comics' Editor-in-chief Jim Shooter:
"Kenner had licensed the DC Heroes. Mattel had He-Man, but wanted to hedge in case superheroes became the next big fad. They were interested in Marvel's characters, but only if we staged a publishing event that would get a lot of attention, and they could build a theme around. Fans, especially young fans often suggested to me 'one big story with all the heroes and all the villains in it', so I proposed that. It flew. Mattel thought that kids responded well to the word "secret", so after a couple of working names bit the dust, we called the story 'Marvel Super Heroes Secret Wars'.'"
Shooter also explained:
"We went through a number of ideas for names for the toy line and series. Mattel’s focus group tests indicated that kids reacted positively to the words “wars” and “secret.” Okay. / Mattel had a number of other requirements. Doctor Doom, they said, looked too medieval. His armor would have to be made more high-tech. So would Iron Man’s, because their focus groups indicated that kids reacted positively...etc. Okay. / They also said there had to be new fortresses, vehicles and weapons because they wanted playsets, higher price point merchandise and additional play value. Okay. / When time came to actually do it, I realized that only I could write it."
Crossover titles included: The Amazing Spider-Man; Avengers; Captain America; Hulk; Iron Man; The Thing; Fantastic Four; Marvel Team-Up; Thor and Uncanny X-Men.
A sequel titled Secret Wars II was published from July 1985 to March 1986.